Why can the tempered glass of the side windows, which is difficult to break with the seat headrest, be easily shattered by a window breaker?

Every summer, typhoons wreak havoc. The accompanying heavy rains flood the city. The rain is pouring down, and if the driver hesitates even a little, the car body may be submerged in water. In this case, it is difficult to open the car door easily because the water pressure on the outside is pressing against the car door. After being soaked in water, the electronic control system is likely to fail, and the car window is difficult to "roll down". At this time, breaking the window is the best choice. The seat headrest is a window-breaking tool recommended in many popular science videos, but due to the small space inside the car, the efficiency of breaking the window is low.

A few days ago, I saw a window-breaking tool that is not a safety hammer. You can easily break the car window glass by pushing it against it. It looks effortless. The car side window glass is also tempered glass. It takes dozens of hits with the seat headrest to break it. But this window breaker can break the glass by just pushing it against it, without hitting it. What's going on?

Let's first understand the mechanical principle of tempered glass breaking. The reason why tempered glass is so strong is that during the processing process, a thin and dense compressive layer is formed on the outer layer, while the inner layer is in a tensile state. When the external load acts on the tempered glass, the compressive stress of the outer layer must be offset first before the glass can be broken. Different processing processes will result in different internal and external tensile and compressive stress values, resulting in tempered glass of different strengths. This "strength" can be divided into two categories, static strength and dynamic strength. For example, the side windows of cars are generally T2 level with a strength of 200MPa, and the front windows of cars are T3 level with a strength of 300MPa. The 200MPa and 300MPa here are static strength. Automobile glass is generally first-level or second-level tempered glass, with impact strengths of 60 and 45J/m2 respectively, and the 60 J/m2 and 45J/m2 here are dynamic strength. Simply put, static strength represents force, and dynamic strength represents energy. When actual damage occurs, the "static" strength is first reached, and the glass is at the critical point of crack initiation, and then the dynamic strength is reached, and the initiated cracks completely spread out.

Whether it is a safety hammer or a seat headrest, or a window-breaking artifact, it is physical destruction and must satisfy the tempered glass breaking principle just mentioned. According to this breaking principle, it is actually very simple to break the tempered glass. It mainly depends on these three factors: the impact speed, the mass of the object, and the contact area. The impact speed and the mass of the object directly determine the kinetic energy of the object, and this part of the kinetic energy can be used to overcome the dynamic strength. The contact area directly determines the stress in the force-bearing area, and this stress can be used to overcome the "static" strength.

The picture comes from Tuchong.com

When the headrest of a seat is used to smash the glass, it is large in size and its mass is dispersed. Although the contact area with the glass is quite small, the impact speed is not enough, and the narrow space lacks the path for acceleration. In addition, the metal rod of the headrest is not specially used for smashing windows, and its material has not been specially treated. The tempered glass is hard, and the metal material is more likely to deform when smashed. Therefore, the efficiency of smashing windows is low. So how does the window-breaking artifact break windows?

I bought one, and also bought a lot of tempered glass. This thing is very small, and the key is that when one end is against the glass, this end will be pressed in, and when it is pressed to a certain degree, the steel needle inside will pop out and shatter the glass.

This window-breaking artifact has two of the three factors mentioned above, namely, impact speed and contact area. Although the artifact itself is very light, its high impact speed makes up for its own quality defects and generates a large amount of kinetic energy. The ejector pin has been specially treated, and its hardness and rigidity are sufficient, so it is not easy to deform. The tip area is small, and it is easy to generate a large stress, thereby breaking the outer compressive stress defense layer of the tempered glass. There is a spring inside the artifact. When we press like this, we transfer energy to the spring and store it. After the pressure reaches a certain level, the mechanism is triggered, the spring is released, and all the elastic potential energy is transferred to the ejector pin and converted into the kinetic energy of the ejector pin. The greater the spring stiffness and the longer the compression stroke, the greater the kinetic energy of the ejector pin, and the thicker the glass can be broken.

Therefore, this window-breaking device relies entirely on the internal spring. In typhoon weather, I hope students pay attention to safety.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Wang Yongjian

Reviewer: Zhou Xiaoliang

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.